I use henna every two weeks (one of the two weeks I add Katam/Indigo to dye the grays).
Wash hair. Afterwards apply coconut oil to my scalp and hair; French braid hair and let it air dry. (I use Vatika frosting from Hairveda.com). I am natural (4Z) and French braid it so that I don't have tangles while applying the henna. My hair is about 4" long.
Henna mix (this is for strength and color; I don't let it sit for 8 hrs when I just want strength):
100g Karishma henna, nettle tea and 1 whole lemon. Mix and let sit for about 8 hrs.
Mix Katam/1 tbsp indigo with sea salt and water. Let sit for 15 to 20 minutes. Mix this solution with 1/2 of the Karishma mix. Freeze the remainder Karishma mix. Apply henna mix and steam for 1.5 hours (I love that I don't have to have it on my hair for 4 to 6 hrs like I used to). BTW my Karishma contains BAQ henna, amla, shikakai, aritha and bhringraj.
Rinse henna out. Co-wash with Ojon conditioner or a cheapie conditioner. I have a lot of Ojon (bought 3-33 oz. bottles some time ago; didn't like it and am trying to use it up by cowashing w/it after I henna).
Afterwards, DC (Usually Hairveda's Sitrinillah, Shescentit.com's Banana Brulee or Bee Mine Products BEE•U•Ti•FUL Moisturizing Deep Cond) with JBCO and Moringa oil added to the DC. Steam for 30 minutes. Rinse DC out. Hair is moisturized.
Apply leave-in (Komaza's Califa Moisturizing spray, KBB Hair Milk, Christine Gant's Sweet Milk detangling leave in or Afroveda's Coconut hair milk), Afroveda's PUR whipped jelly and a hair butter. French braid my hair. Let air dry. Hair is like butta'!
I also use Jamila henna and add ayurvedic powders to it such as amla, hibiscus, brahmi, shikakai or bhringraj powders. And I mix this with nettle tea and lemon juice, too.
I also do the same to my husband’s hair about once every six weeks, but like IDareT's I do a two-step henna and indigo treatment and I don't braid his hair; it's too short. He sits 1 hr. under the steamer w/the henna and 1 hr. under the steamer w/the indigo. His hair is 4A w/some 3C in the back.
I've been hennaing since Sept '09.
One thing I want to try is not let the henna sit to release the color when using Katam/indigo to dye my hair dark brown and see if I still get the same warm, dark-brown color.
HTH